When doing implicit diff. and there are x's and y's it will take the derivitive of x but wipe the y's out. I tried to take the derivitive where the y was dependent on x but it gave me something like y[D[x,y]] something to the effect. but it isn't takig the implicit diff correctly. in the eqn. x^3+y^3+9 x y =0 the 9xy is obliterated. The x would become 1, but the y should return dy/dx yet it does not.